The single speed Brompton wheel has the same hub that is used for the 2-speed. So you could change to a 2-speed by purchasing two cogs, the dearilleur, 2-speed chain tensioner, shifter and cable kit. You'll probably also need a longer chain.
Going to a 3 or 6-speed will also require a new wheel and the appropriate tensioner/shifter/cables, etc.
Brompton offers some of these parts together as upgrade kits.
One of the great things about Brompton is that they make so many parts available to do things like gearing changes/upgrades. However, it can be expensive and mechanically non-intuitive (if not difficult) if you're not used to working on Bromptons. Be sure to check with a dealer before you get started with a gearing change, unless you're confident you know everything such a project would entail.
Best of luck.